Influenza infection remains an important infection in animals and humans. Influenza is caused by viruses that undergo continuous antigenic changes/modifications and that possess an animal reservoir. Thus new epidemics and pandemics may occur in the future, and eradication of the disease will be difficult to achieve. Influenza viruses are well known in the art and described more in detail for example by P. Palese, Nature Medicine, vol. 10, no. 12, pp. S 82 to S 86 of December 2004, with further references. Briefly, the genome of the influenza A virus consists of eight single-stranded segments, and the viral particles has two major glycoproteins on its surface: hemagglutinin (H) and neuraminidase (N). With at least 16 different hemagglutinin (H1 to H16) and 9 different neuraminidase (N1 to N9) subtypes, there is a considerable antigenic variation among influenza viruses.
Influenza virus of type H5N1 Fowl Plague virus has been demonstrated to infect poultry, pigs and man. The viruses can also be transmitted directly from avian species to humans (Claas et al., Lancet 1998, 351: 472; Suarez et al., J. Virol. 1998, 72: 6678; Subbarao et al., Science 1998, 279: 393; Shortridge, Vaccine 1999, 17 (Suppl. 1): S26-S29). Mortality in known human clinical cases approaches about 50%.
Over the last century pigs have been an important vector for influenza pandemics. Pigs, camels, and seals, preferably pigs, can serve as a ‘mixing chamber’ for avian influenza viruses, and therefore represent a potential risk factor for overcoming the species hurdles from poultry, the naturally reservoir of influenza viruses, to mammals. This normally occurs by double infections of the susceptible animals, e.g. pig, with both, an established mammalian (porcine), as well as an avian influenza virus. This double infection may create new recombinant viruses that may be the cause of human or porcine pandemics. Recent evidence would, however, indicate that a recombination of current avian H5 strains with mammalian influenza viruses will not result in highly virulent recombinants. On the other hand, avian influenza virus can infect pigs and by spontaneous mutations can become adapted to pigs. The critical hurdle will be overcome as soon as the virus can cause horizontal infections within a pig (or other mammalian) population.
Yet, a major part of Southeast Asian pigs have been infected with avian (H5) influenza virus strains originating from neighbouring poultry husbandry. As those infections have so far been sub-clinical, they can only be diagnosed by laboratory methods and thus are frequently overlooked. There is a high risk that those sub-clinically-infected pigs will serve as an opportunity for the virus to adapt to the mammalian system, spread within the porcine population, and also infect human beings.
Current influenza vaccines include a subunit vaccine (Babai et al., Vaccine 1999, 17 (9-10):1223-1238; Crawford et al., Vaccine 1999, 17 (18):2265-2274; Johansson et al., Vaccine 1999, 17 (15-16):2073-2080) attenuated vaccine (Horimoto et al., Vaccine 2004, 22 (17-18):2244-2247), DNA vaccine (Watabe et al., Vaccine 2001, 19 (31):4434-4444) and inactivated influenza vaccine (Cao et al., Vaccine 1992, 10 (4):238-242), with the latter being the most widely used on a commercial scale (Lipatov et al., J Virol 2004, 78 (17):8951-8959).
Subunit vaccines, recombinant hemagglutinin and neuraminidase (Babai et al., Vaccine 1999, 17 (9-10):1223-1238; Crawford et al., Vaccine 1999, 17 (18):2265-2274; Johansson et al., Vaccine 1999, 17 (15-16):2073-2080) may be an attractive alternative to the inactivated vaccine, although none are currently in use as commercial vaccines. The preparation of such vaccines is obviously safer than for an inactivated vaccine. Moreover, subunit vaccines do not generate antibody responses to internal influenza viral proteins and thus allow distinction between vaccinated and infected animals (Crawford et al., Vaccine 1999, 17 (18):2265-2274).
Hemagglutinin protein is the receptor-binding and membrane fusion glycoprotein of influenza virus and the target for infectivity-neutralizing antibodies. The entire hemagglutinin protein (HA) from the H5N1 is composed of 568 amino acids, with a molecular weight of 56 kDa. The HA molecule consists of HA1 and HA2 subunits, with the HA1 subunit mediating initial contact with the cell membrane and HA2 being responsible for membrane fusion (Chizmadzhev, Bioelectrochemistry 2004, 63 (1-2):129-136).
Baculovirus/insect cell systems have been used to express hemagglutinin genes isolated from avian influenza subtypes (Babai et al., Vaccine 1999, 17 (9-10):1223-1238; Crawford et al., Vaccine 1999, 17 (18):2265-2274; Johansson et al., Vaccine 1999, 17 (15-16):2073-2080); Nwe et al., BMC Mircobiology 2006, 6 (16):doi:10.1186/1471-2180-6-16). However, those recombinant proteins seem not to be protective in any case, or only less effective at least for some species (Treanor et al., Vaccine 2001, 19: 1732-1737).
The document Lin et al. (J Vet Med Sci. 2008 70 (11):1147-52) discloses the use of a baculovirus/insect cell system for the production of H5 protein of clade 2 H5N1 virus A/duck/China/E319-2/03, which is usable for a prime-booster vaccination for preventing an infection with the clade 2 virus A/duck/China/E319-2/03.
Bright et al. (PLoS One. 2008 30; 3 (1):e1501) describes the use of a baculovirus/insect cell system for generating virus-like particles (VLPs) which include neuraminidase, hemagglutinin and matrix 1 protein from clade 2 H5N1 virus for inducing a cross-clade protective immune response against a challenge with clade 1 H5N1 virus A/VN/1203/2004 in mice. However, the production of VLPs is not without problems, since in order to generate a functional VLP that effectively mimic a real virus, multiple virus structural proteins are needed which must then be correctly assembled into a particle that reproduces the confirmation of the outer shell (capsid) of the infectious virus. Further, study also reals that in vitro assembly of VLPs competes with aggregation (Ding et al. Biotechnology and Bioengineering 107 (3): 550-560).
Thus, there is a need to increase availability of improved vaccines and new vaccination approaches to provide better approaches to control influenza infections and to have a positive impact on disease load. In particular, there is a strong need for a simple, effective and easy-to-handle system inducing, preferably by a single-shot vaccination, a cross-clade protective immune response to influenza viruses with H5N1 HA.